1. Field of the Invention
This invention relates generally to the recovery of hydrocarbons from underground reservoirs, and particularly to the recovery of highly viscous oil and other hydrocarbons that resist flow under normal conditions of temperature and pressure.
2. Description of the Prior Art
The demand for new sources of hydrocarbon fuels, particularly petroleum together with an increase in the price receivable for domestically produced oil, has created renewed interest in increased production of highly viscous petroleum reserves. These reserves make up a substantial portion of the petroleum reserves in the United States.
By "highly viscous" petroleum is meant one having a gravity of 20 or less on the American Petroleum Institute (API) density scale, with lighter crude oils carrying higher numbers than heavier crudes. For example, a typical light density crude may have an API rating of 37 or higher.
A problem with the recovery of these heavy crudes, however, is that the viscosity of the oil, or other hydrocarbon, is so great that the crudes does not tend to flow. Many of these crudes, having API numbers for example, 13 or 14, are essentially tar-like substances. Several systems usually referred to as enhanced recovery techniques, have been proposed for heating the crude while still underground in order to lower the viscosity of the oil and facilitate pumping thereof to the surface below which the reservoir of crude is located. One of the most commonly used process for recovering heavy crude involves injecting steam produced by surface generators into the underground reservoir in order to heat the oil and simultaneously push the crude toward recovery wells disposed near the injection well. This process, sometimes referred to as "steam stimulation", involves locating a steam injection well centrally of, for example, four production wells arranged on a rectangular grid. This process has encountered two basic problems, especially when used to recover oil from pools located at extreme depths-usually well below 2,000 feet. Firstly, the steam generators, which must necessarily be located in the vicinity of the injection wells, create environmental pollution problems. Secondly, the steam cools and liquifies as it proceeds down the base of an injection well, thus effectively limiting the well depths with which the system can be used effectively.
U.S. Pat. No. 3,420,301, issued Jan. 7, 1969 to O. L. Riley, et al, disclosed an apparatus for heating and recovering underground oil. This known apparatus employs a pair of hollow cylindrical concentrically arranged electrodes electrically connected in parallel for the purpose of heating the petroleum by use of a unit containing the electrodes and adapted to be situated in a dielectrically insulated borehole adjacent an oil-bearing formation. The heating unit is capable of allowing water, hot or cold, to be pumped into the oil-bearing formation for changing the viscosity of the oil and having further a capability inherent therein for retrieving the lower viscosity oil. While this prior art device has the advantage of eliminating the necessity for surface steam generators, it is handicapped by being useful for recovering hydrocarbons from only comparatively shallow depths of, typically, 2,000 feet or less, at which limit the cost of supplying electrical energy to the electrodes becomes prohibitive due to inherent inefficiencies in the possible connection of the electrodes and to downhole losses in the leads to the electrodes. These disadvantages of the system employed by this known device result in the operational efficiency of the system being relatively low, especially at greater depths, with 60% to 70% of the BTV input to the system being lost.
U.S. Pat. No. 3,213,942, issued Oct. 26, 1965 to D. C. Nixon, discloses apparatus for removing paraffin buildup in production tubing of oil wells. More specifically, a plurality of arcuate electrodes are arranged in longitudinally spaced relation along a wall of the tubing and are connected to one of an electrical source and electrical ground for causing a current flow through its oil. But, these electrodes, like those in U.S. Pat. No. 3,420,301, discussed above, are connected together in parallel and suffer the same limitations. Further, U.S. Pat. No. 2,350,429, issued June 6, 1944, to D. F. Troupe, discloses a downwell heater in which an electrolyte is heated by passing an electrical current through it from spaced electrodes.
Once again, however, the electrodes are connected in parallel, as they are in the two U.S. Patents discussed above.
Finally, U.S. Pat. No. 2,430,347, issued Nov. 4, 1947, to W. C. Lamphier, discloses a steam generator device in which a current is passed between spaced electrodes for flash vaporizing a liquid disposed between the electrodes. But, like the devices discussed above, this known steam generating device uses electrodes connected in parallel, and therefore would suffer from the same disadvantages as the other devices discussed above if disposed downhole in order to create steam for the purpose of lowering the viscosity of heavy crude.
U.S. Pat. No. 1,726,041, issued Aug. 27, 1929, to D. V. Powell, is mentioned as showing an oil field regenerating system being encased, parallel flat plate electrodes, while U.S. Pat. No. 2,932,352, issued Apr. 12, 1960 to R. J. Steyemeier, discloses a liquid filled well heater using a non-circulating heat conducting fluid. Again, the electrodes of these two devices are connected in parallel. U.S. Pat. No. 1,835,400, issued Dec. 8, 1931, to J. W. Ingeson, et al, discloses an oil well heater with an electrical, closed-circuit heated fluid system, with U.S. Pat. No. 1,464,618, issued Aug. 14, 1923, to R. S. Pershing, disclosing an electric heater for oil wells, and the like, having multiple electrical heating units. Finally, there is known a device as disclosed in U.S. Pat. No. 2,908,331, issued Oct. 13, 1959, to A. L. Brown, which employs a closed-circuit heater-fluid circulating system in an oil well heater. Most of these latter-mentioned devices have the disadvantage of being closed-circuit systems, which do not inject hot liquid or steam (vapor) into hydrocarbon reservoir directly, and these are not as efficient in energy transfer as are the steam injection arrangements.